Off-board navigation guidance systems are distributed computation systems which include a remote navigation guidance device, such as a remote server, for handling the majority of the computational effort and a portable navigation device, such as an in-vehicle device or other portable wireless device, which is wirelessly connected to the remote server. One such system is described in U.S. Pat. No. 6,421,602, assigned to the assignee of the present invention. Conventional navigation systems download route data for only a single route from a route start location to a destination location. When the portable navigation device is moving, its position changes between the time the navigation route is requested and the time the navigation route data is received by the portable navigation device. The portable navigation device may move a large distance during route download, and may in fact pass or have traveled away from or in another direction from the route, thus being located away from the route start location. If this happens, the entire route becomes useless and the portable navigation device must request a new route.
Existing navigation guidance systems may also provide a route from the route start location which may not even be available or accessible to the user of the portable navigation device. This can occur in vehicle applications where the vehicle is in a parking lot or on a school campus and the beginning of the route downloaded starts on a street that is not accessible from the parking lot or campus, or a median prevents the vehicle from turning on a street in the direction given by the route. One solution requires downloading of route data with a tree of various optional routing to initiate the route or to maintain the route. Typically, the amount of data downloaded over the wireless connection is directly related to the cost of providing the route data. Thus, prior art methods which require download of route data with a tree of various optional routing are costly and much of the data transmitted will never be used.
Thus, what is needed is a method for timely and efficiently providing relevant routing data for route initiation and route maintenance. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.